528 MEDICAL ENTOMOLOGY 



moreover, to bear in mind that in the Diptera there are man}' species 

 which are confirmed blood-suckers, but are nevertheless very closely 

 allied to forms which have no biting apparatus at all. The Mallophaga 

 are already parasitic, and as their mouth parts are arranged on the 

 same general plan as that of the rest of the Arthropoda, and have 

 therefore the same potentialities for adaptation to the blood-sucking 

 habit, a common origin for them and for the Anoplura would be in 

 no sense remarkable. For our present purpose it is sufficient to regard 

 the Anoplura as a fairly well-defined order, without going too closely 

 into the question of their affinities. 



The species of lice which infest human beings have long been known 

 to be associated with an uncleanly habit of person, an association 

 which is probably the origin of the feelings of repugnance to which 

 they give rise among civilized people. Their prevalence, in fact, may 

 be taken as an indication of the personal cleanliness of the inhabit- 

 ants of a locality ; in Europe they are much less commonly met 

 with now than appears to be have been the case in the past, while 

 in some parts of the tropics a considerable proportion of the inhabit- 

 ants harbour them. There is never the slightest difficulty in obtaining 

 them in large numbers in Madras. 



The connection between the presence of lice and personal filth has 

 often led to the suspicion of some causal relationship between them 



and diseases, such as typhus and relapsing fever, 

 Relation to disease . . r 



which occur especially in uncleanly surroundings and 



in times of famine. In epidemics of these diseases it is found that 

 it is always the poorest and least cleanly section of the community 

 which suffers most, and indeed it is only when they are brought 

 specially into contact with such people that persons of ordinary habits 

 become affected. Experimental evidence in confirmation of the 

 observed relations has been produced within recent years. Nicolle, 

 working in Algeria, and Ricketts and Wilder, in Mexico, have succeeded 

 in infecting monkeys with typhus fever by means of the bites of lice 

 taken from the bodies of persons suffering from the disease. The 

 latter observers conclude that although the bug and the flea are also 

 under suspicion, all the epidemiological evidence points to the louse 

 as the transmitter; there is a possibility that the infection may be 

 hereditary in the louse. 



With regard to relapsing or recurrent fever, the louse may be said 

 to be now definitely incriminated as the main, if not the only, carrier 

 of the spirochaete which produces the disease. 



